The contents of Application No. EP 94810312.2, filed on May 27, 1994, in Europe, is hereby incorporated by reference.
This invention relates, first of all, to a method for making an electronic module comprising an electronic circuit encapsulated or inset in a body of synthetic material, said body having two essentially flat and parallel main faces, and said module cut according to a predetermined format, in particular in the format of a credit card.
For some years now there have been numerous applications of electronic modules in credit card format having two main faces, i.e. two faces whose dimensions are markedly larger than one of the dimensions (thickness of the card) of the other faces, it being hoped that the two said main faces are preferably flat and parallel, and are equipped with an electronic component including in particular electronic memory means provided with a code, connected to an antenna, such as, for example, the transponder or electronic circuit which can be queried at a distance by inductive means, shown in FIG. 7 of the application EP-A-0 573 469. Such modules can be used, for example, as identification badges.
Other applications require being able to have access to one or more contact tracks disposed generally flush with one of the main faces of the module, said tracks being connected permanently to an electronic circuit disposed inside the module, in order to be able to exchange data with a suitable electronic device equipped also with means permitting contact to be established with the contact track or tracks of the module. Modules of this type are used, for example, as telephone debit cards.
A module equipped with two types of mentioned circuits, either a first access circuit by means of contact tracks and a second access circuit by inductive means allows, in particular, increasing the security of use of the debit or credit card.
Various methods exist for encapsulating an electronic component, with or without coil forming transmitting and receiving antenna such as in the case of the previously mentioned application, inside or on the surface of a body of synthetic material in order to protect it from humidity, from dust and mechanical constraints, said body having in the end a credit card format or any other convenient format so that the device can be easily handled.
A first possibility consists in injecting a thermoformable material around the electronic component; this method requires a costly injection tool and involves difficulties in placing the electronic component correctly inside the body. Moreover, in cases where the electronic component is connected to another component or to a coil, the great pressure exercised by the injected substance can break the very fine wires connecting these elements.
According to another method, the component is disposed between two thermoplastic sheets which are then laminated; even though said component has a sufficiently small thickness, generally less than 0.5 mm, it is difficult to obtain a card having two absolutely flat and parallel faces, a slight swelling always remaining at the location where the component is disposed. To overcome this drawback, it is proposed that, first of all, the component be disposed on the inside of a cutting made in one thermoplastic sheet of a thickness approximately equal to that of the component, then this assembly be laminated between two other thermoplastic sheets. Because of its complexity, this last method is ultimately costly, and moreover, owing to the required dimensional tolerances between the cutting and the dimensions of the component, an empty region or line between these elements can be obtained, resulting ultimately in hollow regions or lines on the main faces of the body.
The application EP-A-0 128 822 presents a method by which an encapsulated electronic circuit, having at least one flat face provided with electrical contacts for subsequent access to a data read/write device is inset in a plate of plastic material, said plate being softened locally to permit insertion of the circuit inside the plate. A major drawback of this method is that the material making up the plate is not compressible and that a swelling is created on the plate's opposite face after the circuit has been inserted, a swelling which requires at least one subsequent operation for its elimination. In another embodiment of the method, a hollow accommodation is provided in the plate for introducing the circuit, the hollow space remaining in the plate after introduction of the circuit being then filled with a filling material. This method is ultimately rather complex, requiring a number of operations, the presence of several materials and creating several transition surfaces between materials.
The application EP-A-0 570 784 describes a manufacturing process where an accommodation is first made in a plate which will then be used as a positioning structure for the electronic circuit. The bonding material in liquid form is then introduced in order to impregnate the plate before pressing. The use of a volatile liquid is a drawback in a manufacturing process since it generally requires large ventilating means. The use of a bonding material in solid form, as is proposed as a variant, requires disposing more layers before pressing. Therefore this metho d also requires supplementary operations, preparation of the accommodation, introduction of the bonding material, and as with all the methods foreseeing an accommodation for the electronic circuit, it can be difficult to ensure a perfect surface evenness of the main faces of the module. Moreover this method is less versatile since it does not allow modules to be manufactured having a circuit whose contact surfaces are flush with one of the faces of the module.
Another drawback of prior art methods is that they call for rather flexible materials, the body remaining flexible when the module is completed. This can be a drawback when the electronic circuit carries a component, in particular a coil of relatively large dimensions; by bending the module it is easily possible to break the coil.
A first object of the invention is thus to propose a method of encapsulating an electronic circuit including at least one electronic component in or on the surface of a body of synthetic material having the shape of a plate, and preferably the dimensions of a credit card, and not having the mentioned drawbacks of the prior art, but permitting a module to be obtained whose two main faces are absolutely flat and parallel, this by inexpensive, easy-to-use means and without it being necessary to precut the sheet or sheets forming the body.
Another object of the invention is to propose an electronic module such as is described above, which does not have the mentioned drawbacks of known modules since it has been made by means of the method according to the invention.
The different objects are attained through different variant embodiments of the method calling for different starting materials all having similar thermal and mechanical characteristics, allowing modules to be obtained according to several configurations of electronic circuits and answering to the features of claims 1 to 20, as well as an electronic module such as is described in claims 21 to 24.